Porosity carbonate rocks

Porosity. The fraction of total (bulk) volume occupied by the voids is defined as the porosity of the porous medium. A porous medium can be classified according to the type of porosity involved. In sandstone and unconsolidated sand, the voids are between sand grains, and this type of porosity is known as intergranular. Carbonate rocks are more complex and may contain more than one type of porosity. The small voids between the crystals of calcite or dolomite constitute intercrystalline porosity (47). Often carbonate rocks are naturally fractured. The void volume formed by fractures constitutes the fracture porosity. Carbonate rocks sometimes contain vugs, and these carbonate rocks constitute the vugular porosity. Still some carbonate formations may contain very large channels and cavities, which constitute the cavernous porosity. 

Carbonate rocks are more frequently fractured than sandstones. In many cases open fractures in carbonate reservoirs provide high porosity / high permeability path ways for hydrocarbon production. The fractures will be continuously re-charged from the tight (low permeable) rock matrix. During field development, wells need to be planned to intersect as many natural fractures as possible, e.g. by drilling horizontal wells. 

The typical value of porosity for a clean, consolidated, and reasonably uniform sand is 20%. The carbonate rocks (limestone and dolomite) normally exhibit lower values, e.g., 6-8%. These are approximate values and do not fit all situations. The principal factors that complicate intergranular porosity magnitudes are uniformity of grain size, degree of cementation, packing of the grains, and particle shape. 

It is possible that greater porosity in shale beds could be achieved by chemical comminution of the shale. For example, the treatment of western oil shales with acid solutions might result in comminution by inducing corrosive stress fracture of the carbonate rock. Chemical engineering research in this area, as well in the elucidation of oil-rock interactions, might provide insights for new strategies for oil shale production. 

The dissolution channels (wormholes), obtained under certain conditions of attack of carbonate rocks by hydrochloric acid, have been recently proven to have a fractal geometry. An equation was proposed, relating the increase of the equivalent wellbore radius (i.e. the decrease of the skin) to the amount of acid injected, in wellbore geometry and in undamaged primary porosity rocks. This equation is herein extended to damaged double porosity formations through minor modifications. 

Carbonate rocks consist mostly of calcite and dolomite with minor amounts of clay. The porosity of carbonate rocks ranges from 20 to 50%, but in contrast to sandstone, it tends to decrease with depth. Often, carbonate rocks are fractured, providing a permeability that is much greater than the primary one. In some cases, initial small-scale fractures in calcite and dolomite are enlarged by dissolution during groundwater flow, leading to an increase in rock permeability with time. 

Let us now consider the problem from the standpoint of calcite precipitation kinetics. At saturation states encountered in most natural waters, the calcite reaction rate is controlled by surface reaction kinetics, not diffusion. In a relatively chemically pure system the rate of precipitation can be approximated by a third order reaction with respect to disequilibrium [( 2-l)3, see Chapter 2]. This high order means that the change in reaction rate is not simply proportional to the extent of disequilibrium. For example, if a water is initially in equilibrium with aragonite ( 2c=1.5) when it enters a rock body, and is close to equilibrium with respect to calcite ( 2C = 1.01), when it exits, the difference in precipitation rates between the two points will be over a factor of 100,000 The extent of cement or porosity formation across the length of the carbonate rock body will directly reflect these... 

Figure 8.35. Porosity versus time-temperature index of maturity (TTI) for various carbonate rock units. The TTI values are based on Lopatin s (1971) method. The onset and end of oil generation (the "oil generation window") in terms of TTI units are from Waples (1980). The log linear equations of best fit between porosity and TTI data are also shown. = porosity. (After Schmoker, 1984.)...

Halley R.B. and Schmoker J.W. (1983) High porosity Cenozoic carbonate rocks of south Florida Progressive loss of porosity with depth. AAPG Bull. 67, 191-200. 

Baumgartner L. P., Gerdes M. L., Person M. A., and Roselle G. T. (1997) Porosity and permeability of carbonate rocks during contact metamorphism. In Fluid Flow and Transport in Rocks Mechanisms and Effects (eds. B. Jamtveit and B. W. D. Yardley). Chapman and Hall, London, pp. 83-98. 

During deeper burial of newly deposited carbonate sediments, the primary and secondary porosity is decreased by cementation and chemical compaction. At these deeper hurial depths pressure solution causes the sedimentary grains to dissolve and cement, and stylolites to form. Stylolites may start to form at depths of 1 to 2 km (Bjorlykke, 1989). Early formed carbonate cement may hamper later pressure solution, i.e. carbonate sediments which have been subject to relatively early cementation may retain their remaining porosity better with depth (Bj0rlykke, 1989). Aqueous dissolution of carbonates may also create secondary porosity in carbonate rocks at deeper burial. The complex evolution of porosity in carbonate sediments and rocks is reflected in the extreme lateral and vertical heterogeneity of carbonate rocks (Mazzullo and Chilingarian, 1992). 

A DOUBLE-POROSITY POROELASTIC MODEL TO RELATE P-WAVE ATTENUATION TO FLUID FLOW IN VUGGY CARBONATE ROCK... 

Class II deposits in reservoirs made up of consolidated terrigenous sediments sands not running reservoirs lying at depths from 800 to 1500 m Class III deposits in reservoirs made up of carbonate rocks with complex porosities of fracture, cavern, and and vuggy types. 

Scholle, P. A., 1979. A Color Illustrated Guide to Constituents, Textures, Cements, and Porosities of Sandstones and Associated Rocks. Amer. Assoc. Petrol. Geol. Memoir 28, Tulsa, 201 pp. Scholle, P. A., 1978. A Color Illustrated Guide to Carbonate Rock Constituents, Textures, Cements, and Porosities. Amer. Assoc. Petrol. Geol. Memoir 27, Tulsa, 241 pp. 

Scholle, P. A., "A Color Illustrated Guide to Carbonate Rock Constituents, Textures, Cements, and Porosities," Memoir 27, American Association of Petroleum Geologists, Tulsa, Oklaboma, 1978, 241... 

Representative values of some physical properties of carbonate rocks are listed in Table 5.32. It can be seen that generally the density of these rocks increases with age, whereas the porosity is reduced. Diagenetic processes mainly account for the lower porosities of the older limestones. The porosity has a highly significant influence on the unconfined compressive... 

A recent paper by Borgwardt and Harvey [79] on the kinetics of this reaction is interesting, not only as a report of an extensive experimental investigation, but also because the conclusions presented are in conformity with the ideas developed in this book. Borgwardt and Harvey characterized eleven diverse types of carbonate rock by a polarizing microscope and scanning electron microscope. The rocks were then calcined, crushed, screened, and subjected to further examination under these microscopes and by mercury penetration porosimeter and by a BET apparatus. An increase in porosity was observed after calcination moreover, there was considerable variation in the mean pore diameter and in the pore volume from rock to rock. The rates of reaction of each of three particle sizes of each calcine with a gas containing 3000 ppm of SO2 at 980°C were then measured and microprobe scans of the reacted calcines were made to determine the location of the absorbed sulfur within the particle. 

In siliciclastic rocks, many physical properties (elastic wave velocity, electrical resistivity, permeability) show a strong correlation to porosity. In carbonate rocks, correlations superimposed by the heterogeneous pore distribution, pore type, pore connectivity, and grain size (Westphal et al., 2005). Table 1.3 compares some prominent properties of the two main groups of reservoir rocks. 

Porosity of carbonate rocks covers a broad spectrum of types and magnitudes as result of a diversity of processes. Lucia (1999, 2007) notes that porosity in carbonate reservoirs ranges from 1% to 35%. The porosity at deposition is high for carbonates (Poelchau et al., 1997). 

Carbonate rocks also show a decrease of porosity under the influence of depth or overburden pressure, respectively. Brown (1997) analysed the influence of carbonate mineralogy, shale content, and fabric on the porosity versus depth correlation. For the study, argillaceous limestone, limestone, dolomitic limestone, and dolomite of the Mississippian Madison Group in the Williston... 

Following the rock-fabric classification, Jennings and Lucia (2001) developed a systematic plot presentation for non-vuggy carbonates and subdivided limestone and dolomite into three classes with rock-fabric numbers. The generalized carbonate permeability model provides a relationship between permeability, interparticle porosity, and rock-fabric number ... 

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